The present invention relates generally to WDM and DWDM communication systems, and more generally to an optical interleaver employed in such systems.
Optical wavelength division multiplexing (WDM) and dense wavelength division multiplexing (WDM) have gradually become the standard backbone networks for fiber optic communication systems. WDM and DWDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM and DWDM technology.
One approach to increasing fiber optic capacity is to use more closely spaced channels. For example, at one point in time, 200 GHz spacing was common for optical channels. At that time optical components were designed to operate on 200 GHz spaced channels. As the state of the art improved, 100 GHz spacing was used for optical channels. Optical components were then designed to operate on 100 GHz spaced channels and devices designed to operate on 200 GHz spaced channels had to be replaced of modified to operate on the 100 GHz spaced channels. This upgrade requirement can be very expensive for parties with an extensive amount of fiber optic equipment that is already deployed.
An optical device that can be used for interfacing between different channel spacing schemes is known as an interleaver/deinterleaver, which is essentially an optical router that allows systems designed for operation at a wide channel spacing to be extended to systems designed for narrow channel spacings. In its simplest form, an interleaver combines two sets of channels into one densely packed set with half the channel spacing. Interleavers/deinterleavers are also used for other purposes, such as to add/drop channels at a node in such a way that one interleaver output adds/drops local channels while the other interleaver output forwards express channels to another node.
One type of interleaver/deinterleaver is based on unbalanced Mach-Zehnder interferometers or Fourier filters, which advantageously can be produced in the form of a planar lightguide circuit An example of an interleaver employing a Fourier filter is a nonlinear Fourier Filter Flat-top (F3T) interleaver such as the WaveProcessor(trademark) Interleaver available from WaveSplitter Technologies. An example of such an interleaver is shown, for example, in U.S. Pat. No. 5,596,661, which is hereby incorporated by reference in its entirety. One problem with an uncascaded Fourier filter interleaver when the channel spacing is less than about 50GHz is that it exhibits a large chromatic dispersion, which degrades system performance. To overcome this problem two or more identical Fourier filters are sometimes cascaded in the same lightpath, but this causes additional performance problems, including an increase in the transmission peak ripple and a narrowing of the effective pass-band width.
Accordingly, it would be desirable to provide an optical interleaver/deinterleaver based on a Fourier filter that has improved performance characteristics.
In accordance with the present invention, a planar lightguide circuit includes a fist pair of planar optical waveguides forming at least three couplers alternating with a delay path between adjacent ones of the couplers. Each of the delay paths include two segments of the planar optical waveguides having unequal optical path lengths. A second of the couplers is of an order different from a first and third of the couplers.
In accordance with one aspect of the invention, the second coupler is a 0th order coupler and the first and the third couplers are 1st order couplers. Alternatively, the second coupler may be a 1st order coupler and the first and the third couplers may be 0th order couplers.
In accordance with another aspect of the invention, the second coupler is located between the first and third couplers.
In accordance with yet another aspect of the invention, a second pair of planar optical waveguides forming at least three couplers alternating with a delay path between adjacent ones of the couplers is provided. Each of the delay paths include two segments of the planar optical waveguides having unequal optical path lengths. A first waveguide in the second pair of optical waveguides having a longest optical path length has an input optically coupled to an output of a second waveguide in the first pair of optical waveguides having a shortest optical path length. A second of the couplers formed by the second pair of waveguides is of an order different from a first and third of the couplers in the second pair of waveguides and from the second of the couplers formed by the first pair of waveguides.
In accordance with another aspect of the invention, a cascaded interleaver, includes at least three two-stage Fourier filters. A first of the Fourier filters includes first and second waveguides such that the first waveguide has a longer optical path length than the second waveguide. A second of the Fourier filters includes third and fourth waveguides such that the third waveguide has a longer optical path length than the fourth waveguide. A third of the Fourier filters has fifth and sixth waveguides such that the fifth waveguide has a longer optical path length than the sixth waveguide. The third waveguide has an input coupled to an output of the first waveguide and the fifth waveguide has an input coupled to an output of the second waveguide. At least one of the Fourier filters include at least three couplers alternating with a delay path between adjacent ones of the couplers. A second of the couplers is of an order different from a first and third of the couplers.